Been a while, but I've been working on a competitive team for the current VGC format, and though its mostly complete, I kinda need some input as to what I could fit into the 6th slot. So far, the team relies on some already proved and efficient cores to deal with just about any kind of threat. In case anyone needs a reminder, the current VGC format is Aloladex only, Alolabred only, Legendary and Mythical Pokemon banned, no mega evolutions, no multiple Pokemon and no multiple items. With that said, the team is as follows:

So, quick run down as what it is about. First and foremost, the team is built around a standard Fire/Water/Grass core. The effectiveness of such a core needs little explanation as it has had presence in the metagame since forever. Right now, it's very popular to run Arcanine/Fini/Kartana (otherwise known as the AFK core). instead of Fini and Kartana, I'm choosing to run Gyarados and Tsareena. Gyarados provides additional Intimidate support to further shut down physical attackers in tandem with Arcanine without the need of burn (which you wouldn't run anyways because Fini is all over the place). Arcanine can also provide Snarl support to suppress opponents from the special side, and this combination of physical and special suppression allows the team to mitigate any kind of offensive pressure the opponent might have.

AV Tsareena is a 'mon that is rarely seen in VGC nowadays, but its ability is something that I've been wanting to play around for a long time. Basically, it's like a one-way Psychic Terrain, which relieves the team of Fake Out & Feint pressure while allowing it to run priority of its own, as well as a different kind of terrain to work with. Tsareena can also take down Gastrodon just as nicely as Kartana can while being somewhat bulkier, and, unlike Kartana, Tsareena can provide Feint support for whenever I really wanna take something down asap. Trop Kick can also provide even more offensive suppression in case double Intimidate isn't enough.

Lastly, Garchomp and Koko are all-around really good Pokemon to have, and they're both kinda needed to help dismantle opposing AFK cores. I'm choosing to run Mixed Koko instead of the standard special attacker not only out of pure novelty, but because Mixed Koko can achieve some relevant damage calcs that Special Koko can't, namely, the ability OHKO the likes of Fini, Celesteela, Mandibuzz, and other specially defensive threats that EV themselves to survive Koko's Thunderbolt. Even with almost no investment, Dazzling Gleam still 2HKO's opposing dragons with Life Orb.

As for the sixth slot, there are four Pokemon I currently have under consideration, but I appreciate any other suggestions you might have. I'm seriously undecided as to which I should pick, so I'd like some help in this regard. These are:

Porygon2 is one of the most used Pokemon in the format, and with good reason. It is bulky af, and has excellent typing due to lack of relevant Fighting types. It has good offensive pressure and it has access to Trick Room, which in this team allows the ability to nullify opposing TRs while also providing a TR mode against teams with hyper offensive and speedy builds. It should be noted Tsareena has really good synergy with P2 in the same vein Lele does as Queenly Majesty prevents T1 Fake Out.

Gluttony Snorlax is also a formidable tank, and it also serves the role of deterring the use of Trick Room by the opponent. Access to Curse allows it to outslow opposing TR threats like A-Marowak.

A-Muk serves a similar role to Snorlax, except it trades a Fighting weakness for a Ground weakness (less desirable given I already have two ground weak Pokemon in the team), but in the process it can better handle opposing Fairies (read: all Tapus).

Scarf Silvally is a novelty option I've been wanting to try out given Parting Shot and the stat supression theme I already have going on. 95 base Atk is not super impressive, but Silvally does have a lot of bulk, and it has access to STAB-Explosion.

Firstly, your Arcanine and Garchomp are somewhat outdated. Burn Up is not as good as the more consistant flamethrower or flare blitz, and snarl doesn't fit on what seems to be an offensive set. I would suggest physical with Wild Charge instead of Snarl, Flare Blitz or Flamethrower instead of Burn up, and the Electrium Z for the surprise burst on Tapu Fini and potentially milotic(though by now people know about Electrium Z arcanine and play accordingly, leading to an ironic situation where arcanine may scare your opponent's water types.)

Your Chomp is also 3 years old, no offense. Dragon Claw and Fire Fang both hit nothing important, and should be replaced with Poison Jab(Tapus and Nintails) and Rock Slide/Stone Edge(Araq/Nintails/Arcanine if you don't want to EQ). Rocky Helmet is also only OK, but being Specially defensive is much better so that enemy Porygon2 don't get Download boosts off your pokemon. Groundium Z is usually a better choice for the burst and Ground coverage without hitting your ally. AV is another option which I'll talk about below

Tsareena is an ok choice if you're into that but I would save the Assault Vest for Garchomp, who likes the bulk, or Metagross, which I recommend as your sixth option. I say this mostly because Tsareena has some pretty good support options, mainly Charm and Screens. Its an option worth considering and I would recommend playing around with the Assault Vest on different mons and trying out supporting options (I would remove play rough for these if you want to try this as that doesn't hit much, only Chomp and maybe Hariyama which trop kick is sufficient)

Tapu Koko on your full physical team should probably be full special, given that it takes less self inflicted damage and isn't weak to intimidate. Thunderbolt Dazzling Gleam Discharge Protect with a Magnet is standard, though you can play around with it a little bit. I would just recommend keeping it special.

For you final Pokemon, I would suggest Metagross to scare off Tapu Lele and Alolan Ninetails, because both of those decimate your team(aside from Arcanine who is fairly threatening to Ninetails). AV Metagross is currently all the rage, but you can probably get away with another item like a 25% berry, air balloon, or maybe a Z crystal (though your team is kinda loaded with those and chomp + canine need it more than Metagross). Band is also something you cna try but band is really only good with Tapu lele for the zen headbutt damage, but you could always try. Meteor Mash, Zen headbutt, and Hammer Arm are the three common damage moves, with either protect or bullet punch/EQ if protect can't be run (AV, Band).

Your team as a whole is offensive and pretty solid at that. As I said before, Metagross deals with Ninetails and Lele, and even if you don't choose to use Metagross a Lele counter is an absolute must as you don't have one which puts you are a pretty big risk to it. Another pokemon which you need to fear is Milotic. Competitive kinda throws your whole team off, so you have to be careful when you see one on the enemy team and play around it. Tapu koko scares it, but if you're forced to send an intimidator in front of a Milotic the competitive and adrenaline orb I believe it can outspeed so watch out for that. It's a little bit unfortunate that Tsareena also fears milotic as trop kick is also going to proc competitive, but if you focus it down early you should be ok. Another smaller threat you should be aware of is Mudsdale, as your team is predominantly physical and so dealing with mudsdale might be difficult. Even with stamina though, your intimidate comp helps, asdoes trop kick, so even with a few defense boosts his damage will be weakened. TrickRoom as a whole is a general threat to a fast team such as yours, but you're decently equipped to deal damage quickly, which means trick room teams will likely be playing on the back foot if you set your leads right. Still somethign to watch out for though, especially towards the end of a match where trick room can become a strong win con

To be honest, after much testing, special tanky Arcanine is much better off with Sitrus Berry & Flamethrower than Burn Up and Firium Z, and is probably the route I'll try next before transitioning into full physical Arcanine. The reason I've kept myself away from physical Arcanine is mostly because, as already noted, the team has a heavy physical bias to begin with. In giving sitrus to Arcanine, I could give Gyarados a Waterium Z instead, as, even when resisted, it packs a much bigger punch when after a DD boost. Hydro Vortex also provides a better answer to Mudsdale, as unboosted it can still get OHKO even after a Stamina boost. I'm also considering Taunt instead of Ice Fang. Mono-Water attacker is not terrible tbh, and Taunt helps deal with Trick Room and other shenanigans a little better. Gastrodon tho... well, it's not like Ice Fang does much against it anyways.

On the subject of Garchomp, I do indeed find Dragon Claw to be not too useful nowadays, especially given the popularity of Tapu Fini, and 'Chomp would likely be better off with Poison Jab instead. However, I disagree on Fire Fang not hitting anything relevant. It hits Kartana & Scizor really well, and it also helps pressure Celesteela whereas Rock Slide does close to nothing to it other than hopefully roll for flinch-hax. For reference other than my own experience, Aaron Zheng has been running EQ/PJ/FF himself rather succesfully. As for choice of item, I found Assault Vest on 'Chomp to be rather underwhelming, and I think I'll find Groundium Z a better choice in order to have a single-target ground-type nuke at my disposition should the need arise. AV Chomp is bulky, yes, but being able to Protect against incoming Ice Beams is much better, imo.

Tsareena with Light Screen is an option worthy of consideration, and I'll try toying around with a Trop Kick/High Jump Kick/Light Screen/Protect set with perhaps Light Clay. I do however also like the immediate bulk AV provides, and I guess it'll ultimately depend on whether or not I find a better AV user.

Lastly, I also agree I lack an obvious answer to Tapu Lele, and in that regards, Metagross and Alola-Muk are the most obvious considerations. Muk also has the benefit of kind of dissuading Trick Room use, especially the Curse set. Given that I just bred a good one, I'll likely experiment with it before trying Metagross. Right off the bat however, I don't like how both of them make the team even weaker to Earthquake, which is an issue that Porygon2 or Snorlax don't really care for, and unless Lele packs Psychum Z, Snorlax can threathen with Heavy Slam. All of this considered, another possibly good idea would be to try out Scizor. AV Scizor can run a similar role to AV Metagross while having a better typing. Iron Head still KO's Lele. I'm thinking of trying an Iron Head/Bullet Punch or X-Scissor/Brutal Swing /Feint set for it. I could try running only Bullet Punch instead of Iron Head, but it falls short of the OHKO without a boosting item besides the whole Psychic Terrain issue to overcome.